His plan to let money follow the child is a moral and political winner.

If Donald Trump knew that promoting school choice would cause such a ruckus on the left, maybe he’d have weighed in sooner. The Republican nominee has found a winning issue by pitching a plan to “provide school choice to every disadvantaged student in America.” Amen.

During a visit to the Cleveland Arts and Social Sciences Academy, Mr. Trump proposed a $20 billion block grant for states by redirecting federal education money to support charter schools and vouchers. He also endorsed merit pay for teachers and said he’d support local candidates who champion school choice.

Most of the $50 billion or so that the federal government spends on K-12 education is targeted to particular programs like teacher training, and rural and STEM education. About $14 billion in Title I funds are earmarked for disadvantaged students. However, this money doesn’t follow kids to private schools, and states often shortchange charter schools.

Mr. Trump wants to let states use federal funds to boost voucher awards, so parents rather than governments get to choose where the money goes. As he noted in Cleveland, “there is no failed policy more in need of urgent change than our government-run education monopoly.” Judging by the panicky reaction on the left, you’d think he’d proposed eliminating public education.

Hillary Clinton said his block-grant plan would “decimate public schools across America.” Yet $20 billion is merely 3% of what states spend on K-12 education each year and less than the increase in school spending in California since 2012. By the way, charters are public schools—freed of union control.

Mrs. Clinton is showing how far left she has moved on education. President Obama has been hostile to vouchers; recall former Attorney General Eric Holder’s efforts to shut down Louisiana’s voucher program that principally benefits poor black kids. But at least Mr. Obama supported charters, while Mrs. Clinton is now openly hostile to these reform public schools.

Unions and their friends are trying to deflect attention from Mr. Trump’s speech and minority outreach by saying the charter school where he announced his plan received a failing grade on Ohio’s school-progress report card last year. But the charter flunked due to a switch in state tests last year that caused student scores to slump nearly everywhere in the state.

In 2014 about 71% of third graders at Cleveland Arts and Social Sciences Academy were proficient in reading. On the new test 55% rated as proficient. Yet the share of students at Cleveland Arts who scored proficient was still more than twice as high as at Harvey Rice Elementary (which has a similar demographic makeup) down the block. That school got an A on student growth.

It’s ironic that progressives are howling about the charter’s performance on standardized tests, which they usually insist are a poor indicator of school and teacher quality. Why is it that the only schools that unions believe should be held accountable for student performance are those run by their competition? That’s a question Mr. Trump should ask from here to November.